Among the various issues that need to be taken into account in the deployment of the Next Generation Network, the clock processing of the Time Division Multiplexed (TDM) signals switched in the packet network nodes is one of the most crucial and critical ones.
Due to the nature of known packet network nodes (e.g. Ethernet) and in particular the non-constant traffic delay through their data switch fabrics there are a number of different synchronization problems that need to be addressed to match the node egress traffic quality requirements. In this regard, reference is made to International Telecommunications Union (ITU) G.707 “Network Node Interface for the Synchronous Digital Hierachy (SDH) and to ITU G.783 “Characteristics of Synchronous Digital Hierachy (SDH) Equipments Functional Blocks”.
Synchronization is required in telecommunication networks in order to meet network performance and availability requirements. Poor network synchronization will lead to large amounts of so-called jitter and wander. Jitter and wander can lead to transmission errors and buffer under/overflow. Both of those faults result in service problems causing high error rates and can lead to service unavailability. Synchronization in TDM networks is well understood and implemented. Typically, a TDM circuit service provider will maintain a timing distribution network, providing synchronization traceable to a Primary Reference Clock (i.e., clock compliance with ITU-T Recommendation G.811). By synchronisation we mean each unit of time of each system clock corresponds to the same, or substantially the same, unit of time as indicated by the reference clock. Network Synchronization requirements must therefore be carefully considered when networks are deployed.
Packet switching was originally introduced to handle asynchronous data. However, for more recent and future applications relating to TDM services the strict synchronization requirements of those applications must be considered. On the other hand, when the TDM services are carried over a packet network or only switched through packet network nodes data fabric switches some critical aspects arise. The so-called Packet Delay Variation (PDV) introduced by a packet network and/or by packet network nodes data fabric switches is one of the main problems that needs to be addressed. PDV comes about as a result of congestion, internal spreads and different flows passing through the fabric.
FIG. 1 shows a known network 1 arranged to handle TDM traffic which is to be conveyed over a Packet Switched Network (PSN). In the scheme an edge node 2 is at the edge of both a PSN (data) network and of a PSTN (Public Switched Telephone Network, i.e. TDM) network. Counterpart edge nodes 8 and 9 are located on the other sides of the networks. In this very flexible application extensions of the edge node ingress interfaces can accept a variety of different traffic types (e.g. Data interfaces [e.g. Ethernet], TDM traffic [e.g. SDH/SONET/POS/PDH and so on]) and to apply them a flexible number of processes in both the PSN and the PSTN networks. The edge node 2 comprises a plurality of input traffic cards on a receiving side of the node, and a plurality of output traffic cards on a transmitting side of the node and a plurality of data switch fabric cards therebetween. The switch fabric cards connect the outputs of the input cards to the inputs of the output traffics cards.
In the PSN bound direction the data interfaces can be required to be classified, policed, switched/routed, scheduled and so on, and the TDM interfaces can be required to be “Circuit Emulated”, i.e. segmented and encapsulated into data packets to the PSN network, or can be required to be TDM terminated in order to extract the embedded data payload (e.g. Ethernet) to be processed in the same way of the native data interfaces (e.g. classified, policed and so on).
In the PSTN bound direction the ingress data interfaces can be mapped into a TDM frame (e.g. Ethernet over SONET) and the TDM ingress interfaces can be monitored and cross-connected (switched). In this latter case of TDM ingress interfaces, TDM cross-connections and TDM egress interfaces to the PSTN network, the TDM requirements must be taken into account in the configuration of the edge node.
Whilst some hybrid systems comprise both a data switch fabric and a TDM switch fabric, state of the art data switching fabric equipment comprises a single high performance data switch fabric system able to switch all the different traffic types. This space and cost optimization allows maximization of the number of the traffic slots available in the physically constrained rack size and to obtain a cost-effective and more flexible equipment.
The expression ‘switch fabric’ is generally understood to include data processing equipment that is configured to move data coming into a network node (the ingress traffic) out by the correct port (i.e. egress traffic) to the next node in the network.
From the synchronization effects point of view one of the greatest differences between a typical TDM traffic switch fabric system and a data traffic switch data system is the fact that a TDM switch fabric is capable of performing all of the required cross-connection functions with a constant latency while the typical behaviour of a data traffic switch data system is characterized by a low, but non-zero, delay variation (i.e non-constant latency). From this point of view the requirement to have TDM egress interfaces in compliance with the international recommendations synchronization requirements (e.g. the ITU-T requirements) are in this case much more challenging and requires complex and expensive filtering and cleaning functions in the egress cards of the edge node to compensate or mitigate synchronisation impairments (i.e. variations in the data frequency) introduced by the data fabric.
Considering again the arrangement shown in FIG. 1, an egress TDM interface 7 is composed of the same components (e.g. SDH Virtual Containers Vc3, Vc4 and so on) collected from different ingress TDM interfaces 4, 5 and 6 via a cross-connection function of switching fabric equipment of the edge node 1. The data frequencies of traffic of the ingress TDM interfaces are not synchronous among themselves but rather are characterized by (slightly) different frequencies, albeit within the constrained frequencies accuracies. This is due to causes including network noise, network impairments and component precision.
After a segmentation process is performed on the ingress traffic, the packetised traffic is cross-connected across the node by the switching fabric to an appropriate output card where it is then reassembled into TDM form for transmission to the PSTN network. This is the so-called SaR, or Segmentation and Reassembling, process, performed by the edge node, the different containers are each characterized by a respective ingress frequency modified by FDV of the switch fabric.
The task to clean or substantially eliminate this effect is significant and expensive but is nevertheless mandatory in order to meet the egress TDM (e.g. SDH STM-1) synchronization requirements; (in the example SDH case, for instance, the FDV contribution can cause undesired and unacceptable burst Vc pointer re-justification movements).
Considering the general position, an edge node has N TDM input interfaces each one characterized by a nominal frequency fnom, typical of the particular TDM interface and speed and an actual instantaneous frequency fin1, . . . , finN within the recommended frequency accuracy Δfin.
Therefore, there are N non-synchronous TDM interfaces with frequencies:fin1(t)=fnom+Δfin1(t)Δfin1(t)<Δfin fin2(t)=fnom+Δfin2(t)Δfin2(t)<Δfin finN(t)=fnom+ΔfinN(t)ΔfinN(t)<Δfin 
In this general scenario each TDM frame is composed of M asynchronous multiplexed flows (the case of M synchronous flows can be considered a particular case of this general scenario).
The egress filtering process (i.e. the process to ensure that the frequency of the TDM traffic output is substantially that of the node's (regulated) local clock) can be described as a function that receives f1+Δ1; f2+Δ2; . . . ; fn+Δn as inputs (with f1, . . . , fn and Δ1, . . . , Δn all unknown quantities) and provides f1+Δ1f; f2+Δ2f; . . . ; fn+Δnf as outputs with the aim to minimize the Δ1f; . . . ; Δnf terms.
The processing task for the egress cards of the edge node to take account of effects of both FDV and the various different frequencies which were received by the ingress cards so that the egress cards output TDM flows which have the required (system) data frequency is an onerous one.
We have realised that it would be desirable to adopt a new approach to address the issue of switching TDM traffic through a data switch fabric and provide a method to minimize synchronization impairments in the egress TDM traffic signals.